I. Field of the Invention
This invention relates to a process for preparing silicon carbide platelets, to the platelets thus formed and to the use of the platelets for particular applications.
II. Description of the Prior Art
Silicon carbide in the form of whiskers, i.e. single crystals grown primarily in one dimension, can be used as reinforcing materials for matrices made of various materials, particularly ceramics and metals. However, silicon carbide whiskers are suspected of being a health hazard because, like asbestos fibres, they easily become airborne and can be ingested by humans or animals. There is therefore a need for an alternative to silicon carbide whiskers that offers similar reinforcing effects without the associated risks.
Silicon carbide platelets have come under consideration as alternatives to whiskers for matrix reinforcement. Silicon carbide "platelets" are single crystals of SiC having two dimensions appreciably greater than the third. Generally, platelets have an aspect ratio (i.e. length or width to thickness) greater than 3 and preferably greater than 5. Unfortunately, platelets produced by the conventional method of reacting silica and carbon at high temperature are generally highly agglomerated and cannot easily be separated. Agglomerated platelets are not very useful as reinforcing materials because they remain in clumps in the matrix and are difficult to disperse. Moreover, the resulting platelets tend to be thicker than desired for an optimum reinforcing effect.
For silicon carbide platelets to be useful in applications such as the reinforcement of ceramic or metal matrix composites, it is believed that they must possess specific attributes, namely:
(a) they must be substantially completely unagglomerated; PA0 (b) the size should preferably be less than 50 .mu.m, and more preferably less than 30 .mu.m; PA0 (c) the aspect ratio (width/thickness) should desirably be greater than 5 (preferably 5-8), at a diameter of 10-20 .mu.m.
In commonly-assigned co-pending U.S. Pat. No. 5,080,879 (the disclosure of which is incorporated herein by reference), a process is described for producing single platelets of SiC suitable for matrix reinforcement. The process involves the reaction of particles of a non-graphitizable form of hard carbon (e.g. Pennsylvania anthracite) containing 0.5-1.5% by weight of aluminum and at least 0.2% by weight of iron, with silica or a silica precursor at a temperature in the range of 1900.degree.-2100.degree. C. under an inert atmosphere. If the carbon contains 0.2-1.0% by weight of iron, 0.1-10% by weight of boron, relative to the weight of SiO.sub.2, is required in the reaction mixture (if not already present as an impurity in other starting materials) in order to ensure proper growth of platelets. The resulting particles have an average size of less than 50 .mu.m and an aspect ratio greater than 5. While these particles have proven to be very useful for the intended purpose, most of the platelets (about 97%) have a size greater than 20 .mu. m and I have now found that an even more effective reinforcing effect can be obtained if the majority of the platelets have a size of less than 20 .mu.m.
There is therefore a need for an improved process for producing smaller unagglomerated silicon carbide platelets in high yield.